US 3728785 A
The disclosure herein pertains to methods for fabricating discrete semiconductor devices, particularly light-emitting diodes. The disclosure more particularly concerns a diffusion process to form controlled regions of P-type conductivity in N-type conductivity semiconductors.
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Description (OCR text may contain errors)
United States Patent 9] Schmidt 11] 3,728,735 51 Apr. 24, 1973 FABRICATION OF SEMICONDUCTOR DEVICES  Inventor: John G. Schmidt, St. Louis, Mo.
 Assignee: Monsanto Company, St. Louis, Mo.
 Filed: Apr. 15, 1971  App]. No.: 134,251
OTHER PUBLICATIONS Electronics, June 12, 1967, pages 8290, Gallerim Arsenide FETs Outperform Conventional Silicon MOS Devices.
Primary Examiner-Charles W. Lanham Assistant ExaminerW. Tupman Attorney-William I. Andress, Neal E. Willis and J D. Upham 5 7 ABSTRACT The disclosure herein pertains to methods for fabricating discrete semiconductor devices, particularly lightemitting diodes. The disclosure more particularly concerns a diffusion process to form controlled regions of P-type conductivity in N-type conductivity semiconductors.
6 Claims, 11 Drawing Figures Patented April 24, 1913 3,728,785
I m I a FIGQB Q w .7
FIG. [0 l9 Y INVENTbR H JOHNGSCHMIDT Q BY (Lg! M/utw ATTORNEY art describes BACKGROUND OF THE INVENTION This invention pertains to the field of semiconductor devices, particularly light-emitting devices, and fabrication methods therefor.
As pertains to one aspect of this invention, the prior numerous methods for fabricating semiconductor devices wherein conventional photolithographic techniques are used in conjunction with various masking, impurity diffusion and etching systems to provide one or more regions of one conductivity type in semiconductor bodies of another conductivity type. By variation of these techniques simple or complex semiconductor components may be fabricated to produce a variety of electronic devices, including light-emitting devices.
Among the various diffusion systems described in the prior art are vapor phase, solid phase and liquid phase diffusions of the conductivity-type determining impurity into the masked or unmasked semiconductor substrate body to provide active regions therein. Some of the diffusions described in the prior art must be conducted in evacuated and sealed ampoules (closed tube diffusion), while others may be performed as an opentube diffusion.
With respect to various diffusion masking systems, it is known to use a layer of SiO or impurity-doped SiO through which, or through windows of which, certain impurities may be diffused into the semiconductor wafer or to use an impurity-doped Si or SiO layer from which the impurity is diffused into the semiconductor substrate. See, e.g., US. Pat. Nos. 3,255,056, 3,352,725, 3,450,581, 3,502,517, 3,502,518 and 3,530,015. It is also known to use diffusion masks ofsilicon nitride which may be further coated with silicon (U.S. Pat. No. 3,537,921) or metals (U.S. Pat. No. 3,519,504) which are deposited in direct contact with a surface of the semiconductor body. Another masking/diffusion system involves masks havingseparate, distinct portions consisting, respectively, of various oxides, e.g., SiO and laminated Si N /SiO or SiO /Si N /SiO this latter type of combination mask has been described (U.S. Pat. No. 3,484,313) in connection with a selective diffusion process for diffusing a plurality of different types of impurities into different regions of a semiconductor body, each portion of the mask being effective to block or partially block specified impurities; the system is said to be suitable for gas phase, solid phase or liquid phase diffusions.
Problems commonly encountered in most prior art diffusion systems include poor control and reproducibility of the impurity surface concentration, diffusion profile, junction depth and planarity of the P-N junction. Still other problems relate to masking systems used; for example, lack of adhesion of the mask to the semiconductor surface; permeability of the mask to the in-diffusing impurity and/or out-diffusion of volatile constituents or desired impurities in intermetallic or elemental semiconductors, thus requiring very thick or heavily-doped masking layers; reactivity of the masking material with the impurity and/or semiconductor body and necessity to use a closed-tube diffusion with some masking system.
Therefore, it is an object of the present invention to provide a unique diffusion system for fabricating semiconductor devices.
More particularly, it is an object of this invention to provide a solid-solid open-tube diffusion process which overcomes the above-mentioned problems associated with diffused semi-conductor devices.
Still more particularly, it is an object of the present invention to provide a diffusion system which is controllable, simple and economical.
These and other objects will become apparent from the detailed description given below.
SUMMARY OF THE INVENTION This invention relates to a unique impurity diffusion system to fabricate semiconductor devices; in preferred embodiments, full chip emitter discrete light-emitting diodes (LEDs) are provided from III-V compounds or mixtures thereof.
The semiconductor device fabrication process herein comprises the use of an impurity diffusion system consisting of an SiO /ZnO/SiO- sandwich-structure diffusant source, which is in intimate contact with the semiconductor body of N-type conductivity, to provide a means of controllably diffusing zinc into the full surface thereof. Upon heating the structure, zinc is diffused from the diffusant source to form a region of P- type conductivity in the N-type semiconductor substrate body. When the semiconductor component is an arsenide of the Group III element, the component, having protective layers of SiO thereon, is heat-treated to anneal the interface between the diffusion surface of the component and the SiO layer incontact therewith and simultaneously densify the latter forming a diffusion modulation layer.
BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1-6 are cross-sectional schematic views of a semiconductor wafer during successive steps in the fabrication of one embodiment of an LED.
FIG. 7 is a top plan view of another embodiment of an LED fabricated according to this invention.
FIG. 8 is a cross-sectional schematic view taken along line A-A' of the completely fabricated LED shown in plan view in FIG. 7.
FIG. 9A is a top plan view showing a plurality of LED chips with metal contacts attached; FIG. 98 representing the contact position on a single chip.
FIG. 10 is a cross-sectional schematic view taken along line B-B of the LED shown in FIG. 9B.
DESCRIPTION OF PREFERRED EMBODIMENTS The present invention in its preferred embodiments relates to a method for fabricating full chip emitter discrete light-emitting diodes (LEDs). Preferred semiconductor materials include gallium arsenide (GaAs), gallium phosphide (GaP) and gallium arsenide phosphide (GaAs P where X is a numerical value greater than zero and less than 1 EXAMPLE 1 In a preferred embodiment of this invention, LEDs are prepared with gallium phosphide as the semiconductor component of the device.
Referring to the drawings, which show successive stages in the fabrication process of this embodiment, FIG. 1 represents a cleaned and polished GaP wafer l in cross-section schematic view. The GaP is of N-type layer 2 of SiO about 500 A. thick is deposited on the front (top) surface of'the GaP substrate wafer 1; the SiO layer may be prepared and deposited by various means known to the art and in this example, by reacting silane (SiH with oxygen carried by nitrogen at temperatures of from 300400C to deposit SiO on the GaP wafer. A layer 3 of zinc oxide (ZnO) about 350 A. Thick is then deposited on SiO layer 2 as shown in FIG. 3. The ZnO layer is formed and deposited by reacting diethyl zinc, carried in nitrogen, with oxygen at about 400C or, generally, within the range of from 300500C. A final layer 4 of SiO about 1,000 A. thick is then deposited over the ZnO layer as shown in FIG. 4. The SiO layer tends to retard out-diffusion of zinc from the ZnO layer. The wafer thus prepared is then transferred to an open tube diffusion furnace and heated to 875C in forming gas for 30 minutes. Zinc is diffused from the ZnO layer through the SiO layer 2 into the substrate wafer to form a graded P region 5 I (FIG. 5) approximately 1-2 microns below the surface which has a surface zinc concentration of about 3X10 atoms/cc.
It will be apparent that the diffusion times and temperatures may be varied with a variation of the thicknesses of the ZnO and/or SiO layers, zinc concentration and junction depth of the P region and semiconductor substrate material.
After the diffusion operation the cooled wafer is then treated in aqueous HF or an aqueous mixture of HFzNl'hF for a time, less than a minute, sufficient to etch away the SiO /ZnO/SiO diffusant layers (2, 3 and 4) shown in FIG. 4 and leave the Zn-diffused P/N structure shown in FIG. 5. The wafer is rinsed with deionized water (DI), then lightly etched in hot (80C) HCl for about 3 minutes, rinsed again with DI then with isopropyl alcohol (IPA) and dried. The wafer is back lapped to a thickness of 5-6 mils and cleaned.
After the wafer has been cleaned, contacts and leads are attached thereto. Ohmic contact is made to the N surface by vacuum evaporating a Au/Ge alloy (12% Ge) layer 6 onto the back side of the wafer l. The wafer is then attached N-side down to a post or header (not shown). Contact to the P surface 5 is made by bonding conductive wire, e.g., Au, lead 7 directly to the surface of the GaP wafer; this wire bond may be made by any suitable means such as thermo-compression bonding or ultra-sonic bonding.
The device thus prepared is then encapsulated with an appropriate lens for LED devices, e.g., clear epoxy.
EXAMPLE 2 In another preferred embodiment of this invention, LED's are prepared with gallium arsenide phosphide (abbreviated to GaAsP for compositions in the general formula GaAS P where X is greater than zero and less than one as the semi-conductor component of the device. The phosphorus content preferably is from 30-50percent and in this example, the composition is approximately GaAs P The GaAsP component for the device to be fabricated may be processed as a wafer of GaAsP or as an epitaxial film thereof grown on a compatible substrate of GaAs. In either case, the fabrication steps will generally be the same as those described above for GaP LED devices, except for the modification noted below, reference being made to FIGS. 1-5 where applicable.
FIG. 1 represents a cleaned and polished GaAsp wafer 1 in cross-section schematic view. The GaAsP is of N-type conductivity doped with tellurium to a carrier concentration of about 5 l0 atoms/cc, or generally, within the range of about I I05 10 atoms/cc. In FIG. 2, a layer of Si0 (not shown) about 1000 A. thick is deposited on the back (bottom) surface and a layer 2 of SiO about 200 A. thick is deposited on the front (top) surface of the GaAsP substrate wafer 1; these Si0 layers may be prepared as described in Example I. The wafer is now heat treated at about 875C or, generally, within the range of from 800950C, in forming gas for about 1 hour. This 'is a highly important step, involving annealing of the SiO /GaAsP interface as well as forming a densified modulating layer 2 for the subsequent diffusion of zinc therethrough, thus providing further control of the zinc diffusion into the GaAsP wafer. This step in the process is not necessary when the substrate material is GaP.
Following the heat treatment, a layer 3 of zinc oxide (ZnO) about 300 A thick is deposited on layer 2 as shown in FIG. 3. The ZnO layer is formed and deposited by reacting diethyl zinc, carried in nitrogen, with oxygen at about 400C or, generally, within the range of from 300500C. A final layer 4 of SiO about 500 A. thick is then deposited over the ZnO layer as shown in FIG. 4. The SiO layer tends to retard out-diffusion of zinc from the ZnO layer. The wafer thus thicknesses of the ZnO and modulating SiO, layers,
zinc concentration and junction depth of the P region and semiconductor substrate material.
After the diffusion operation the cooled wafer is then treated in aqueous HP or an aqueous mixture of l-IFzNILF for a time, less than a minute, sufficient to etch away the SiO, layer on the back of the wafer and the SiO /ZnOISiO diffusant layers (2, 3 and 4) shown in FIG. 4 and leave the PIN structure shown in FIG. 5..
This structure is then cleaned with sequential treatments with hot l-lCl, DI, isopropyl alcohol (IPA) and dried.
After the P region is formed, aluminum is then vacuum evaporated to a thickness of LOGO-1,500 A over the front surface (9 in FIG. 8) of the wafer forming the P contact of the GaAsP wafer. Using photomasking and etching, the aluminum metallization pattern 10 is defined on the LED device as shownin FIG. 7; FIG. 8 is a cross-sectional view of the device taken along the line A-A of FIG. 7.
After the P-surface contact has been made, ohmic contact is then made to the back (N surface 8 in FIG. 8) by any suitable means. A preferred ohmic contact method is disclosed and claimed in copending application, U.S. Ser. No. 21,637, filed Mar. 23, 1970 and assigned to the assignee of this application. That method involves vacuum evaporating first a layer of tin, then a layer of gold onto the N-surface, heating the wafer to alloy the tin and gold with a surface region of GaAsP to form an N region 11 therein as shown in FIG. 8; a layer of nickel 12 is then electroless plated onto the N region followed by electroless plating a layer of gold 18 to the nickel. Alternatively, the tin, gold, nickel and gold layers may be first deposited then all four alloyed together with a surface region of GaAsP to form the N region 11 therein. Thereafter, the device is attached, N side down, to a post or header (not shown), a wire lead 14 bonded to the aluminum bonding pad 100, e.g., as shown in FIGS. 7 and 8 and then encapsulated in a suitable lens (not shown) for LEDs, e.g., clear epoxy. As with the GaP device described in Example I, light from this GaAsP device is emitted through the P surface.
EXAMPLE 3 In still another embodiment of this invention, LEDs are prepared with GaAs as the semiconductor component of the device.
In this example the GaAs semiconductor is of N-type conductivity doped with silicon to a carrier concentration of about 3.5 l atoms/cc or, generally, within the range of about l l0 to X10 atoms/cc. The fabrication process described in Example 2 is followed, except that the diffusion is conducted for about 7 hours and the P/N junction depth is about 15 microns. Also modified are the ohmic contacting procedures. Ohmic contact is first made to the N surface 16 in FIG. by vacuum evaporation thereto of a Au/Ge alloy (12% Ge). In FIG. 9A is shown a plurality of GaAs dice, (LED chips) ah, fabricated on a single wafer with the Au/Ge contacts 18 and 21 formed by photoresist and etching techniques well known to the art. The wafer is then scribed and cleaved into individual die, one of which is shown in FIG. 98, showing die C in FIG. 9A. Next, the P surface contact is made by vacuum evaporating a layer of Au/Zn alloy 19 over the P surface 17. The wafer is then attached, P surface down to a header and a conductive lead wire 20, such as Au or Al, is bonded by thermocompression or ultra sonic bonding to bonding pad 18a as shown in FIG. 10. Light from this LED device is emitted through the N layers as depicted by the wavy arrows.
The preferred embodiments of the invention described herein are by way ofillustration only, and not limitation. Other semiconductor materials in the III-V which comprises:
a. providing an N-type semiconductor substrate selected from the group consisting of GaAs, GaP and GaAs,, P where X is a number from zero to one inclusive;
. depositing a layer of SiO over the back surface of said substrate when it is GaAs or GaAs P and another layer of Si0 over the front surface of said substrate;
c. heat treating the structure of step b when said substrate is GaAs or GaAsP;
d. depositing a layer of P-type impurity oxide onto said layer of SiO deposited on the front surface of said substrate;
e. depositing a layer of SiO onto said layer of impurity oxide;
f. heating the structure of step e to diffuse impurities into said semiconductor substrate and form a region therein of P-type conductivity;
g. removing said SiO and impurity oxide layers from the front surface of said semiconductor substrate;
h. applying the necessary ohmic contacts and electrical leads to saidsemiconductor substrate and i. encapsulating the device.
2. Process according to claim 1 wherein said impurity oxide is ZnO.
3. Process according to claim 2 wherein X in said formula equals one and said semiconductor substrate is Ga? and step c is omitted.
4. Process according to claim 2 wherein X in said formula equals zero and said semiconductor substrate is GaAs.
5. Process according to claim 2 wherein said semiconductor substrate is GaAs P where X is a number greater than zero and less than one.
6. Process according to claim 2 wherein the semiconductor device is a light-emitting device and is encapsulated in transparent material.